Phil Appleton (NHSC/IPAC) : Warm H2 Emission in Compact Groups of Galaxies: A New Insight into Group Evolution to the Red Sequence

Observations by our team of Stephan’s Quintet (HCG 92) with Spitzer, have shown the radio/X-ray filament to emit strongly in the pure-rotational lines of warm molecular hydrogen suggesting excitation in shocks. After briefly reviewing new CO and optical spectroscopy of SQ showing broad and peculiar line profiles in this intergalactic shock, I will discuss a more comprehensive mid-IR spectroscopic study of 23 Hickson Compact Groups (78 galaxies). The results show a peculiar connection between the enhanced warm H2 emission and both morphological type and NUV-IR color. Galaxies with high warm H2/PAH ratios (suggestive of shocks) in the HCGs studied lie in the optical “green valley”, and are almost all lenticular (S0) galaxies. They also lie in a narrow zone in specific star formation rates (sSFR) between “red-and dead” ellipticals, and active star-forming spirals, suggesting they are a transition population. We discuss several possible ways in which these galaxies have been transformed in a dense environment, from enhanced tidal and collisional stripping, to late-stage HI infall. Future observations with Herschel and with large-format optical IFUs will help to confirm whether the warm H2-enhanced green-valley galaxies are shock-heated objects.